1. Field of the Invention
The present invention relates to a pulsed laser, or more particularly, to an excitation circuit for a laser.
2. Description of the Related Art
FIG. 13 is a circuit diagram showing an excitation circuit for an excimer laser or a conventional pulsed laser disclosed, for example, in "Actualities of Shortwave Laser Technologies" furnished in Technological Report of the Association of Electricity. Part II, No. 217, p. 5. In FIG 13, 1 and 2 are a pair of mutually-opposing main discharge electrodes. 3 denotes a peaking capacitor connected in parallel with the main discharge electrodes 1 and 2. 4 denotes a pulse generation capacitor. One terminal of the pulse generation capacitor 4 is connected to the main discharge electrode 1. 5 denotes a switch connected between other terminal of the pulse generation capacitor 4 and the main discharge electrode 2. In this example of prior art, a thyratron is employed for the switch 5. 6 represents a charge reactor connected in parallel with the peaking capacitor 3. Then, 7 is a charge terminal connected to other terminal of the pulse generation capacitor 4.
Next, the operations of a conventional excitation circuit will be described in conjunction with FIG. 14 showing waveforms representing voltage and current in components. When positive high voltage is supplied to a charge terminal 7, a pulse generation capacitor 4 is charged through a charge reactor 6. When a switch 5 closes at a time t.sub.0, the charge stored in the pulse generation capacitor 4 is delivered to a peaking capacitor 3. At a time t.sub.1, discharge starts between main discharge electrodes 1 and 2. For an excimer laser, main discharge must be preceded by preliminary ionization discharge. The electrodes and circuits for preliminary ionization discharge will not be described. After discharge starts, the peaking capacitor 3 feeds energy to a discharge induced between the main discharge electrodes 1 and 2. This causes the laser to oscillate. For the excimer laser or other laser having low discharge resistance (for example, 0.2 ohms), voltage across the peaking capacitor 3 represents an oscillatory wave. A voltage Vr of reverse polarity developed at a time t.sub.2 as shown in FIG. 14.
As described above, voltage of reverse polarity develops across the peaking capacitor 3. This reverse main discharge current (Ir in FIG. 14). As a result, a luminescent spot on which discharge concentrates is likely to occur in the surfaces of the main discharge electrodes 1 and 2. Consequently, the surfaces of the main discharge electrodes are heated in spots to evaporate. This shortens the service lives of the electrodes 1 and 2. In particular, when oscillation is repeated at a high speed, this luminescent spot occurs frequently. This accelerates exhaustion of the electrodes and cripples uniform discharge. Eventually, laser output energy per pulse deteriorates. The reverse cycle of the aforesaid main discharge current I is as short as 30 ns. No measures have taken over electric circuitry in the past.